Diode Testing

On the display you should get a 3 or an infinite voltage reading depending upon the multimeter used

Connect the red probe to the cathode and the black
probe to the anode of the diode

The display will immediately show the low forward
voltage drop (rectifier diode) of around 0.6 volts

Now reverse the connections, the display will return to its
original reading indicating that the diode is a good one

If the meter displays any other reading, the diode may be
leaky or faulty while a 0000 reading means a short

There are other methods, but this is about the simplest method. Depending on the type of multimeter you are using. Other methods that can be used is the resistance test method ie testing the resistance in the forward and reverse biased manner.